1. Field of the Invention
The present invention generally relates to heliostats and more particularly concerns heliostats which include a cylindrical reflecting surface for focusing light on the elongate aperture of an elongate solar energy receiver.
2. Description of the Prior Art
U.S. Pat. No. 3,892,433 to Blake discloses a "point" type of central receiver system which includes a central light receiver and a fan-shaped array of heliostats surrounding the central receiver. Each heliostat includes a reflecting surface which is spherical and upwardly concave. The curvature of the reflecting surfaces of each heliostat is fixed and is chosen, among other parameters, according to the distance between the heliostat and the central receiver. The reflecting surface of each heliostat is mounted for pivoting orthogonally about a vertical axis through its center, to thereby permit general elevational and azimuth tracking of the sun. It is indicated in this patent that the heliostats do not accurately focus sunlight into the aperture of the receiver throughout the day. Rather, some inaccuracy is inherent in the system due to off-axis astigmatic errors in both focal planes of the reflecting surfaces which result from the fixed radius of curvature of the individual reflecting surfaces and the fixed mode of orthogonal rotation.
A similar point type of central receiver system, which has been studied by the Georgia Institute of Technology, is disclosed at pages 69-73 of the publication entitled, "Total Energy, Distributed Collector Systems and Research and Development Projects" published by the Aerospace Corporation of El Segundo, Calif. on Mar. 26, 1976 for the United States Energy Research and Development Administration in connection with ERDA Contract No. E(04-3)-1101. The system as described therein includes heliostats arranged in a hexagonal field about the central receiver. Each heliostat includes a flat circular mirror bent into a fixed curved form by a bending device, which device is not specifically described. The individual mirrors are, as in the system of the aforenoted Blake patent, held at fixed radii dependent upon their respective distances from the central receiver. As with the heliostats of the Blake patent, the heliostats disclosed in this publication will also be subject to focusing errors resulting from off-axis astigmatism.
U.S. Pat. No. 3,861,379 to Anderson discloses a receiver system, which may be broadly considered to be a small scale, line type of central receiver system. In a line type of central receiver system, the receiver is elevated relative to a field of heliostats and is elongated along an axis which is oriented in a selected direction. In the line type of system the heliostats have cylindrical or rectangular reflecting surfaces which are mounted for rotation about axes which are parallel to the axis of the central receiver. The system of the Anderson patent includes a central cylindrical receiver fixedly mounted to a framework and a plurality of parallel, elongate reflectors, each having flat rectangular shapes, which are rotatably mounted to the framework. The patent indicates that the reflectors may have cylindrical or parabolic cross-sections.
U.S. Pat. No. 3,884,217 to Wartes shows a distributed receiver system which is similar to the system of the Anderson patent. The Wartes patent discloses a mechanism for rotating the mirrors thereof into defocused positions in the event that the mirrors need protection from the elements (hail, sand, etc.) or when the central receiver is in an overheated condition.
U.S. Pat. No. 3,915,147 to Rineer discloses a system in which two rigid reflector members, which together form a trough-shaped reflector, are shifted into a defocused orientation with respect to the associated receiver when said receiver needs to be cooled below the condensation point of the heat transfer fluid therein.
As indicated by the Rineer, Blake, and Wartes patents, the conventional method of providing defocusing of incident light comprises changing the relative orientation of rigid reflectors relative to the receiver, rather than bending the reflectors themselves into defocused configurations.